estimation of serpentinite rock mass strength of placetas ... · in a principal substratum folds in...

FUOYE Journal of Engineering and Technology, Volume 2, Issue 1, March 2017 ISSN: 2579-0625 (Online), 2579-0617 (Paper)

FUOYEJET © 2017 89 engineering.fuoye.edu.ng/journal

Estimation of Serpentinite Rock Mass Strength of Placetas - Cuba Underground Gold Mine Deposit

1Oluwaseyi Adeoluwa Olajesu and

2*Ajibola Olawale Olarewaju

1Instituto Superior Minero Metalúrgico, Facultad de Geología y Minería, Departamiento de Minas, Moa, Cuba 2Department of Chemical Engineering Technology, University of Johannesburg, South Africa

and Department of Materials and Metallurgical Engineering, Federal University Oye Ekiti, Nigeria

[email protected]|[email protected]

Abstract— This study estimated the strength of the serpentinite rock mass of the underground gold mine “Oro Descanso” Placetas, Cuba. The rock mass was classified into its lithological group of massive, sheared serpentinite rocks and gabbros. The geo-technical information from the well log data obtained during drilling process (geological logs). The structural analysis was carried out through field observation and quantified by Geological Strength Index (GSI) of average values for massive serpentinite 60, sheared serpentinite 38 and gabbros 78. The generalized Hoek-Brown criterion with software programme, Rocklab 1.0, 2004 version was employed for the analysis and the determination of the rock mass local compressive strength (massive serpentinite = 1.733Mpa; sheared serpentinite = 0.464Mpa; gabbros = 10.354Mpa) and the global strength (massive serpentinite = 6.561Mpa, sheared serpentinite = 5.657Mpa and gabbros = 22.547Mpa). These estimated values characterize brittle type of failure mode and thus supports are recommended. Keywords: Rock mass, Strength, Underground mine

—————————— ——————————

1 INTRODUCTION

he Cuba gold mineralisation is wide spread, occuring

as alluvia and endogenous deposit. Available report

shows that they are classified as Au-Ag deposit with Sb

veins; quartz-Au sulphides with chalcopyrite. There are

several distinct mining districts where gold has been

mined with over 400 mining companies operating in Cuba

(Figure 1). Many gold mines are located near Santa Clara

in central Cuba. The Placetas underground mine is not as

famous as some other dated back to the prehistiric era in

Cuba or early Spanish conquest [Gold in Cuba – Mining

and Prospecting Areas (Rare Gold Nuggets June 28, 2015)]

Much geotechnical information on ore deposit is required

in designing prior to mining and extraction.

Reports (Qui et al.., 2017) show that the mechanical

properties of transversely isotropic rocks have attracted

much research interest in the past years (Cho et al., 2012;

Dan et al., 2013) for the anisotropic behavior exhibited by

this type of rock (Vervoort et al., 2014) and also for huge

number projects built on these rocks. Thus it is necessary

to understand the anisotropic behavior of these rocks,

such as the exploitation of shale gas, (Harris et al., 2011),

roof support design of transversely isotropic rock, (Lee et

al., 2008) and excavation of anisotropic rocks in

underground tunnels (Zhang and Sun, 2011).

The estimation of the strength and strength parameters of

the rock mass of any underground mine demands a high

level of reliability. The strength of any rock mass depends

on many factors like: strength of the intact rock, the

*Corresponding Author

condition of discontinuity, water inflow, anisotropy,

homogeneity, etc. In order to include all these parameters

for a reliable estimate of the rock mass is a complex task

and many a times need the complex and costly state of art

measuring instruments (Zhao et al, 2010) and which are

not affordable by most third world countries, but many

researcher had proposed empirical and theoretical

methods which have receive worldwide acceptance due to

their practical approval. Barton (1973) and Barton and

Choubey (1977) proposed empirical method for the

estimation of shear strength of rock mass, Maksimovlc,

(1996) proposed hyperbolic relation which does not need

any empirical assumption in order to determine shear

strength of rock mass as compared to Barton’s. Hoek and

Brown (1980), Hoek (2007) proposed semi-empirical

method for the determination of rock mass strength,

which is the method that is applied, in this study, to

estimate strength of the rock mass of Oro Descanso Mine.

Fig 1: Gold-Projects-Cuba-HSBC-Nov-18-2016. (Source:

Sierra Geological Consultants Inc)

T



1.1 Geological Characteristics of the Deposit

The study area is located in the municipal of Placetas, Villa

Clara, Cuba, with coordinates points according to Lambert

system: A(274300,628000), B (274300, 628450), C (273850,

628450) and D (273850, 628000). Geologically, it is located

in a principal substratum folds in the central region of

Cuba where a complex rock mass is found of the

continental nature, oceanic nature type ophiolitic and with

different mixture of earth types. The deposit is found

within complex ophiolitic rocks located in wild form over

the sedimentary sequence of the continental bank and at

the same time over run by volcanic cretaceous arc.

(Orestes et al. 2010). The principal rock mass type is

massive serpentine with veins of gabbros. The mineral

occurrence is associated with the tectonic zone conserved

within massive serpentine wedge. Serpentinite is formed

from olivine via several reactions between the

magnesium-endmember forsterite and the iron-

endmember fayalite. In the reaction there is exchange of

silica between forsterite and fayalite thus forming

serpentine group minerals and magnetite as represented

in equations (1-3).

3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2 (1)

Fayalite Magnetite aqueous silica

3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4 (2)

Forsterite aqueous silica serpentine

2Mg2SiO4 + 3H2O → Mg3Si2O5(OH)4 + Mg(OH)2 (3)

Forsterite serpentine brucite

The zone is affected by systems of faults of orientation

between 250º-285º and dip within 65º-90º, also there exist

transverse fractures with little development along its

length, all these provoke displacement generally not more

than 0.2m.

1.2 Generalised Method of Hoek and Brown

This criterion was obtained through the curve of best fit of

the experimental data of rock failure drawn on a principal

stress plan σ1-σ3 and as one of the few techniques available

for the estimation of the strength of rock mass with the aid

of geological data, it is based on the assumption that the

rock mass consists of a sufficient number of joint sets (at

least three), such that the rock mass behaves as an

isotropic material, such rock masses are interlocked, but

the interlocking level is relatively low, as joints are

persistent and therefore sliding on block boundaries

dominates failure, with some rotation of intact rock pieces

(blocks)., this criterion is often used for analysis in rock

mechanics (Cartaya and Blanco, 2000; Bahrania and

Kaiser, 2013). Hoek and Brown (2002) proposed the

following expression (4) for the determination rock mass:

a

ci

bci sm

3

31

(4)

where:

1 , and 3 are principal effective stresses.

σci is the compressive strength of the intact rock.

s and a are constants which depend on the rock mass

characteristics.

mb is a reduced value of material constant

mi is the constant of the intact rock which is determined by

statistical analysis of triaxial values of principal stresses or

through chart (Hoek, 2007).

The values of mi and σci are obtained by statistical fit to

peak strength data within a confinement range of 0 to

0.5sci. The empirical constants mb and s are related in a

general sense to the angle of internal friction of the rock

mass and the rock mass cohesive strength respectively, a,

controls the curvature of the failure envelope. The

parameter a, is typically near 0.5 for high GSI-values (>55)

and reaches 0.6 for extremely poor ground (Han et al, 2012;

Bahrania and Kaiser, 2013)

Initially, this criterion was developed for the analysis of

fractured but unaltered rock mass with resistant and hard

intact rock, supposing that the blocks of rock are

interlocked and that the strength of the rock mass depends

on the strength of the discontinuities (Hoek and Brown,

1980).Therefore, this failure criterion is valid for isotropic

rock mass and do consider the factors that determine the

large scale failure of rocky medium, factors like: no

linearity of a certain level of stresses, the influence of the

types rock, the relation between the compressive and

traction strength, the reduction of frictional angle with the

increase in confining stress, etc. The strength of a jointed

rock mass depends on the properties of the intact rock

pieces and also upon the freedom of these pieces to slide

and rotate under different stress conditions. This freedom

is controlled by the geometrical shape of the intact rock

pieces as well as the condition of the surfaces separating

the pieces. Angular rock pieces with clean, rough

discontinuity surfaces will result in a much stronger rock

mass than one which contains rounded particles

surrounded by weathered and altered material (Bahrania

and Kaiser, 2013). The quantitative value of Geological

Strength Index (GSI) was introduced by Hoek (1994, 2007),

through which the quality of the rock mass is estimated in



function of the level and characteristic fracturing of the

rock mass, its geological structure, block sizes and

condition of discontinuities, but Sonmez and Ulusay

(1999, 2002) amended this by introducing chart which

included the structure rating, SR, based on volumetric

discontinuity frequency, introduced to describe the

rock mass structure and the surface condition rating,

SCR, estimated from roughness, weathering and

infilling conditions, to describe the discontinuity

surface conditions (Zhang, 2005; Shen et al 2013). The

values of mb, s and a are determined by the following

equations (5-7):

(5)

(6)

(7)

D is a factor which depends on level of disturbance by

blasting and stress relaxation.

The constants 28 and 9 in Eqs.(2) and (3) are called the

degradation constants, as they control the reduction rate

of mb and s as a function of GSI. The uniaxial compressive

strength, σc, and tensional stress, σt are estimated by

equations (8) and (9) respectively

(8)

(9)

2 METHODOLOGY

The geo-technical information from the well logging data

obtained during process drilling process based on visual

inspection of samples brought to the surface (geological

logs) and validated by the physical measurements made

by instruments lowered into the hole (geophysical logs).

The Well logging record of rock mass with the description

of the lithology is presented in Table 1.

Plate 1 shows pictorial view of the Oro Descanso -

Placetas, Cuba underground mine deposit. The mine was

lithological zoned into three major rock type namely

massive serpentinite, sheared serpentinite and gabbros.

The density, humidity and compressive strength tests of

the intact rocks were carried out using 20 to 30 numbers of

54 mm diameter core samples. The tests were performed

at three laboratories (Geominera Mining Company,

Hidráulicos’ Company and Recursos Company) in Santa

Clara, Cuba. The average result of these properties was

estimated by t-student statistical method at the probability

of 0.95.

Table 1: Summary of Well Log Record of Rock Mass.

Plate 1: Oro Descanso - Placetas, Cuba underground

mine deposit (photograph date 2015-02-12)

The quantitative value of GSI was estimated using the

Sonmez and Ulusay’s chart (Sonmez and Ulusay, 1999,

2002). Likewise the values of mb, a and s. are determined

using equations 2-4. The values of D based on the

disturbance level (during blasting at Oro Descanso Mine)

and the values of mi for massive serpentinite, sheared

serpentinite and gabbros were estimated using the Hoek’s

chart (Hoek, 2007). The data was processed with the aid of

Rocklab 1.0, 2004 computer program from which the

estimated values of rock mass strength and its strength

parameters are obtained.

3 RESULTS

Table 2 shows the values of compressive strength of

saturated intact rocks of the rock mass, the values of the

density and humidity. The values of mi, D, surface

structures, roughness, weathering and the filling materials

were determined based on the data obtained from the

field. Using the charts of Sonmez and Ulusay (1999, 2002);

Zhang (2005) and Hoek, (2007). The GSI values were

determined for each rock type (Table 3). Also, the values of



mb, s and a were determined using equations (2, 3 and 4)

while the local strength equations for each rock mass type

are presented in Table 4. Plate 2 shows the section through

the deposit showing different rock aggregates.

a

b

Plate 2: Sections (a,b) through the deposit showing

different rock aggregates of Oro Descanso - Placetas,

Cuba underground mine deposit

Figures 2 to 4 were obtained through computer program,

Rocklab 1.0, 2004 using data values of unconfined

compressive strength of intact rock, mi, GSI, and D. The

graphical plots of principal stresses (σ1-σ3); mb, s and a,

from the Hoek-Brown criterion; as well as the cohesion

and friction angle were obtained from Mohr-Coulomb

criterion.

Table 2: Physical Mechanical Properties of Massive

Serpentinite

Table 3: Estimation of GSI by the Method of Sonmez and Ulusay (1999, 2002)

Table 4: Oro Descanso Rock Mass Strength Estimation Equation

Fig 2: Analysis of Oro Descanso massive serpentinite rock

strength using Rocklab 1.0, 2004)



Fig 3: Analysis of Oro Descanso sheared serpentinite rock strength using Rocklab 1.0, 2004)

4 DISCUSSIONS

The strength of rock mass depend on various factors like

the mineral components of the intact rock, the strength of

the rock grains internal bonding force, cohesive force, the

level discontinuity and its binding force, water content,

etc, that is why the values of compressive strength (σci) of

massive serpentinite, sheared serpentinite and gabbros

(Table 2) were reduced from 31.97Mpa, 46.63Mpa,

54.96Mpa to 1.733Mpa, 0.464Mpa, 10.354Mpa respectively

in the rock mass (Figures 2-4), also, the deposit zone has

been affected by fault, past tectonic activities and constant

minor seismic disturbances which do occur in central

province of Cuba. The humidity of the mine is between

0.33-0.44, this is partly due the fact that the water inflow is

low. The knowledge of the state of the GSI (Table 3) local

strength of the rock around the excavations, the global

strength and modulus of deformation of the rock mass

(Figure 2-4) will aid in the design of the excavation,

selection of proper support method and mining system so

as to avoid the danger of the falling of small pieces of rock

during mining activity. Generally, the values mb, ranges

from 0.98 to 8.88, s from 1.8. 10-4 to 4.34. 10-2 and a, from

0.501 to 0.514 the significance these variations of the

material constant of the rock mass due to different

lithology in the rock mass, and this values are of great

importance in the numeric analysis of the rock mass which

is beyond the scope of this paper. The Mogi’s line (green)

in Figures 2-4, which is defined by the ratio of the

principal stress as σ1/σ3 = 3.4 is generally found below the

principal stress failure envelop (red), this means that the

failure mode that will occur in Oro Descanso rock mass

will be brittle type.

5 CONCLUSIONS

Applying the generalized empirical criterion of Hoek-

Brown, the local strength, global strength and modulus of

deformation of Oro Descanso underground rock mass

were determined which could be effective data for the

design of the mine support, excavation design and for the

selection mining system. Also, the material constants of

the mine rock mass were determined and the equations

that relate the principal stresses were established. The type

of failure mode will be brittle. Hence the needed support

are designed and recommended.

6 ACKNOWLEDGEMENT

The first author, A.O. Oluwaseyi appreciates TETFUND for the sponsorship of the programme

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estimation of serpentinite rock mass strength of placetas ... · in a principal substratum folds in...

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